1. Field of the Invention
The present invention relates to a communication system, a mobile terminal and a transfer device.
2. Description of the Related Art
Studies have recently been conducted on a mobility management system called a hierarchical mobile IPv6 (HMIP). According to this control system, there is a transfer device (mobility manager) which performs control to so that a packet can reach a mobile terminal even if a mobile terminal moves. The transfer device executes transferring or buffering of a packet addressed to the mobile terminal. According to the control system, there are a plurality of access router devices, connected to the mobile terminal, and a plurality of transfer devices at different places in a network. Thus, the mobile terminal need to figure out a transfer device closest to the access router device connected to the mobile terminal in the network. Incidentally, a second device close to a first device in the network means a device to which the first device can quickly access through the network.
Therewith, the access router device transmits a router ad (router advertisement) to the mobile terminal connected thereto. This router ad (AR) contains information regarding a transfer device closest to the access router device in the network (e.g., information stored in a MAP information option). Then, the mobile terminal acquires the information regarding the transfer devices to figure out the transfer device closest to the access router device connected to the mobile terminal in the network (see “Hierarchical Mobile IPv6” (IETF Internet-draft), (online). Internet (URL http://www.ietf.org/internet-drafts/darft-ietf-mobileip-hmi pv6-0.5. txt)).
However, the aforementioned conventional technology has the following problems. To enable transmission of the router ad (router ad containing information regarding the transfer device closest thereto in the network) by each access router device, the following setting is necessary in the access router device. That is, information (e.g., address) regarding the transfer device closest to each of all the access router devices in the network must be set in the access router device.
Thus, for each installation of a new transfer device or a new access router device in the network, in the access router device, it is necessary to perform processing for setting information regarding a transfer device closest thereto in the network. Consequently, a load imposed to maintain the communication network becomes large.
Additionally, a fixed relation is established (fixed relation caused by the aforementioned setting) between the access router device and the transfer device. When a transfer device is added or deleted in the network, the fixed relation must be changed to enable transmission of appropriate information regarding the transfer device to the mobile terminal by the access router device. Consequently, it is necessary to change setting of all the access router devices close to the transfer device in the network.
Furthermore, if there is an access router device in which no information regarding a transfer device is set, a mobile terminal connected to the access router device cannot figure out a transfer device closest thereto in the network.